Field of the Invention
The present invention relates to ophthalmic lenses, and more particularly, to contact lenses designed to slow, retard, or prevent myopia progression. The ophthalmic lenses of the present invention comprise a high plus or high add treatment zone, thereby preventing and/or slowing myopia progression.
Discussion of the Related Art
Common conditions which lead to reduced visual acuity are myopia and hyperopia, for which corrective lenses in the form of spectacles, or rigid or soft contact lenses, are prescribed. The conditions are generally described as the imbalance between the length of the eye and the focus of the optical elements of the eye. Myopic eyes focus in front of the retinal plane and hyperopic eyes focus behind the retinal plane. Myopia typically develops because the axial length of the eye grows to be longer than the focal length of the optical components of the eye, that is, the eye grows too long. Hyperopia typically develops because the axial length of the eye is too short compared with the focal length of the optical components of the eye, that is, the eye does not grow enough.
Myopia has a high prevalence rate in many regions of the world. Of greatest concern with this condition is its possible progression to high myopia, for example greater than five (5) or six (6) diopters, which dramatically affects one's ability to function without optical aids. High myopia is also associated with an increased risk of retinal disease, cataracts, and glaucoma.
Corrective lenses are used to alter the gross focus of the eye to render a clearer image at the retinal plane, by shifting the focus from in front of the plane to correct myopia, or from behind the plane to correct hyperopia, respectively. However, the corrective approach to the conditions does not address the cause of the condition, but is merely prosthetic or symptomatic.
Most eyes do not have simple myopia or hyperopia, but have myopic astigmatism or hyperopic astigmatism. Astigmatic errors of focus cause the image of a point source of light to form as two mutually perpendicular lines at different focal distances. In the foregoing discussion, the terms myopia and hyperopia are used to include simple myopia or myopic astigmatism and hyperopia and hyperopic astigmatism respectively.
Emmetropia describes the state of clear vision where an object at infinity is in relatively sharp focus with the crystalline lens relaxed. In normal or emmetropic adult eyes, light from both distant and close objects and passing though the central or paraxial region of the aperture or pupil is focused by the crystalline lens inside the eye close to the retinal plane where the inverted image is sensed. It is observed, however, that most normal eyes exhibit positive longitudinal spherical aberration, generally in the region of about +0.50 Diopters (D) for a 5.0 mm aperture, meaning that rays passing through the aperture or pupil at its periphery are focused +0.50 D in front of the retinal plane when the eye is focused to infinity. As used herein the measure D is the dioptric power, defined as the reciprocal of the focal distance of a lens or optical system, in meters.
The spherical aberration of the normal eye is not constant. For example, accommodation (the change in optical power of the eye derived primarily though change to the internal crystalline lens) causes the spherical aberration to change from positive to negative.
As noted, myopia typically occurs due to excessive axial growth or elongation of the eye. It is now generally accepted, primarily from animal research, that axial eye growth can be influenced by the quality and focus of the retinal image. Experiments performed on a range of different animal species, utilizing a number of different experimental paradigms, have illustrated that altering retinal image quality can lead to consistent and predictable changes in eye growth.
Furthermore, defocusing the retinal image in both chick and primate animal models, through positive lenses (myopic defocus) or negative lenses (hyperopic defocus), is known to lead to predictable (in terms of both direction and magnitude) changes in eye growth, consistent with the eyes growing to compensate for the imposed defocus. The changes in eye length associated with optical blur have been shown to be modulated by changes in both scleral growth and choroidal thickness. Blur with positive lenses, which leads to myopic blur and decreases scleral growth rate, results in hyperopic refractive errors. Blur with negative lenses, which leads to hyperopic blur and increases scleral growth rate, results in myopic refractive errors. These eye growth changes in response to retinal image defocus have been demonstrated to be largely mediated through local retinal mechanisms, as eye length changes still occur when the optic nerve is damaged, and imposing defocus on local retinal regions has been shown to result in altered eye growth localized to that specific retinal region.
In humans there is both indirect and direct evidence that supports the notion that retinal image quality can influence eye growth. A variety of different ocular conditions, all of which lead to a disruption in form vision, such as ptosis, congenital cataract, corneal opacity, vitreous hemorrhage and other ocular diseases, have been found to be associated with abnormal eye growth in young humans, which suggests that relatively large alterations in retinal image quality do influence eye growth in human subjects. The influence of more subtle retinal image changes on eye growth in humans has also been hypothesized based on optical errors in the human focusing system during near work that may provide a stimulus for eye growth and myopia development in humans.
One of the risk factors for myopia development is near work. Due to accommodative lag or negative spherical aberration associated with accommodation during such near work, the eye may experience hyperopic blur, which in turn stimulates myopia progression as discussed above. Moreover, the accommodation system is an active adaptive optical system; it constantly reacts to near-objects, as well as optical designs. No matter what optical designs one puts on the eye, when the eye accommodates to near-objects, continuous hyperopic defocus will be present and make the eye myopic. Therefore, one way to design optics to slow the rate of myopia progression is to utilize a high plus signal to the retina through use of high add or plus powers.
U.S. Pat. No. 6,045,578 discloses that the addition of positive spherical aberration on the contact lens will reduce or control the progression of myopia. The method includes changing the spherical aberration of an ocular system by a direction and degree related to alter the growth in eye length, in other words emmetropization may be regulated by spherical aberration. In this process, the cornea of a myopic eye is fitted with a lens having increasing dioptric power away from the lens center. Paraxial light rays entering the central portion of the lens are focused on the retina of the eye, producing a clear image of an object. Marginal light rays entering the peripheral portion of the cornea are focused in a plane between the cornea and the retina, and produce positive spherical aberration of the image on the latter. This positive spherical aberration produces a physiological effect on the eye which tends to inhibit growth of the eye, thus mitigating the tendency for the myopic eye to grow longer.
Although the level of positive spherical aberration and/or plus power required to achieve an optimum slowdown in the myopia progression rate is unclear, researchers in the field have attempted to use multi-zone devices with regions of positive power of about +1.50 to a maximum of +3.00 D add in an attempt to slow the progression of myopia. The approach resulted in treatment results of less than about 50 percent. Treatment efficacy is defined as the relative change of axial length and/or spherical equivalent refraction from baseline for a test group compared to the change of axial length and/or spherical equivalent refraction of a control group over a year or a predetermined time period. There remains a need for a myopia control treatment with efficacy greater than 50 percent and closer to 100 percent. Intuitively adding treatment zones of high plus power would provide greater treatment as the ocular growth response in animals was proportional to the power of the optical stimulus as reported by Wildsoet, Vision Research 1995.
However, conventional wisdom in the field of bifocal or multifocal ophthalmic lenses assumes lenses with high plus or high add power may have deleterious effects on vision and contrast sensitivity as reported by Ardaya et al, Optometry 2004. Further, Smith et al (U.S. Pat. No. 7,025,460) teaches against going to powers outside the range normally found in bifocal or multifocal lenses for presbyopia. They state “It is important to note that, while the appropriate type of refractive defocus can drive eye growth (or non-growth) leading to myopia (or its regression) in the phenomenon of lens compensation, when the amount of refractive defocus is great, there may be such a large degradation in image quality due to the severe defocus that the optical state may change into the phenomenon of form deprivation and may induce myopia in that way.” Further, they teach “that the maximum amount of relative curvature of field before substantial vision degradation occurs, which leads to form deprivation myopia, to be around the spherical equivalent of +3.50 D to +4.00 D, which represents the upper limit for negative curvature of field for effective treatment of myopia.” This belief has discouraged researchers from pursuing high plus treatment zones for myopia control.
To the contrary, applicant's research shows that using a design with a central distance zone and a high plus or high add treatment zone having a plus power greater than about 3.00 D reduces visual acuity loss relative to low conventional plus type designs with no significant additional impact on contrast sensitivity. This is also supported in recent work by De Gracia et el, OVS 2013, although they only investigated up to 4.00 D of add power and did not relate the work to a potential benefit in myopia progression control. This breakthrough enables ophthalmic designs to achieve a meaningful greater than 50 percent slowdown in myopia progression without further negatively impacting visual acuity.
Further, significantly higher plus power relative to the distance power is not expected to lead to reduced accommodation as may occur with a lower add power design where a subject might rely to some extent on the add power for clear vision during near work activities, as has been observed during the course of our research. This reduced accommodation may lead to hyperopic defocus of rays passing through the distance portion of the device. In the current invention, the subject must accommodate over the distance portion of the lens for near vision correction as objects imaged through the treatment zones of high plus powers are sufficiently out of focus that they cannot be cleared with the accommodation-convergence system.
Another researchers in the field, R. Griffin WO2012/173891, claims to relieve accommodative lag and accommodative stresses that lead to myopia progression through the creation of an artificial pinhole that results in increased depth of focus and depth of field. In their intellectual property, “the eye's accommodation is more relaxed” in contrast to the present invention.
With reference now to FIG. 1, the graph illustrates a device with a design that incorporates a distance zone to correct for distance vision and a peripheral zone of variable plus power. Visual acuity was measured using a four forced choice method with progressively smaller Snellen optotypes. Increasing peripheral plus power to about +2.00 D to +3.00 D causes an increasing loss of high contrast visual acuity, as typical of multifocal type designs for presbyopes. As the peripheral power continues to increase; however, the relative effect on visual acuity surprisingly improves and plateaus, so that by above about +4.00 D to +5.00 D peripheral plus, the visual acuity loss becomes relatively constant. This is of significance for the design of myopia control lenses, since higher plus power is found (with animal models) to have a greater impact on eye growth, as reported in Wildsoet, Vision Research 1995.
However, further optimization of plus power designs is required to optimize image quality. With reference now to FIG. 2, power profiles are illustrated having +5.00 D or +10.00 D power beyond a 2.25 mm radial location from a center of a lens. Rays passing through these high plus or high add power regions form sharp foci in front of the retina. However, due to continued propagation to the retina, these rays form a ring-like defocus blur on the retina.
As shown in the point spread function (PSF) cross section of FIG. 3, rays coming from the +5.00 D and +10.00 D regions form separate spikes on the retina. Thus, if one looks at a point light source through one of these +5.00 D or +10.00 D high plus lenses, his/her retina would receive a peak signal surrounded by a ring-like halo. Usually, this is not a problem when one reads letters or resolves fine details of objects because the halo is so dim that the human doesn't perceive it. Nevertheless, this is a problem if a person looks at a black/white edge, as energy from the white background can leak into the black due to the presence of the spike in PSF.
With reference now to FIG. 4, the image cross section for the +5.00 D and +10.00 D power profiles of FIG. 2 at an entrance pupil size of 6.0 mm are shown by convolving the PSF with the black/white edge in object space. A lens having 0.00 D power forms a sharp edge between the black and white (at 0.0 mm location) and thus doesn't have a ring-like structure. On the other hand, the lenses with +5.00 D and +10.00 D regions do not have a sharp edge between black and white, thereby resulting in images in which the black background is not completely black, and the white background is not completely white.
Accordingly, the presence of halo is an inherent property of high plus or high add lens designs. The present invention is directed to lenses having high plus power treatment zones that are suitable for the use in treating, controlling, or reducing the progression of myopia while also minimizing a halo effect.